1. Field of the Invention
The present invention relates to an image forming apparatus, image forming method, and a computer-readable storage medium in order to stabilize output features of an output apparatus.
2. Description of the Related Art
In recent years, print-on-demand (hereafter called POD) using a high-speed multi-function device has become common for printing work and copying work. POD efforts have advanced in general industries in order to self-manufacture meeting materials or proposals, and in design offices in order to perform prepress or printing of color comprehensive layout. Workflow of digital printing has permeated, with these industries centrally, and the usability and economy has becomes widely understood, and a market called “digital business printing” has been formed. Printing apparatuses applicable to this market not only outputs high quality printed material, but are printing apparatuses securing high production.
An electrophotography method is an image recording method used with an image forming apparatus such as a photocopier. Electrophotography is a method which forms a latent image on a photosensitive drum using a laser beam, and develops this with a charged color material (hereafter called toner). Image recording is performed by transferring an image exposed with the toner to a transfer sheet and fusing thereupon.
Nowadays, the electrophotography method is not only employed as printing apparatuses for complex machines in offices, but has also come to be employed as to printing apparatuses to create quality printed materials and so forth in the “digital business printing” market.
With the electrophotography method, calibration processing is a process performed in order to keep the output image at a predetermined density quality. Calibration processing is processing performed in order to reduce the density of a printed image due to shift in output density or changes in coloring at the printing apparatus, owing to change over time. A general calibration processing operation of the electrophotography method will be described with reference to the drawings.
FIG. 11A is an operation flow diagram to describe a calibration operation. FIG. 11B is a diagram showing calibration performing timing. As shown in FIG. 11B, the calibration operation with the electrophotography method is generally executed with a flag input timing from the printer engine employing the electrophotography method or a interrupt signal timing (T1, T2, T3) as to a control system. In many cases, the flag, or the timing that the interrupt signal is output, is at the point-in-time that printing of a predetermined number of sheets has been performed. For example, in the case that the printing apparatus is such that performing density correction of the output image is better at the point-in-time that the number of sheets printed has reached 500 sheets, a flag, or an interrupt signal, is generated from the output engine each time printing of 500 sheets has ended.
Next, the execution flow for calibration will be described with reference to FIG. 11A. Upon a predetermined number of sheets being printed, the output engine outputs a flag/interrupt signal as to a printing apparatus control side (hereafter called controller) (step S401). Upon receiving the flag/interrupt signal, the controller instructs the output engine to output patch image data which the engine has (step S402). A patch image is image data of a predetermined size and predetermined density defined for each engine, and is generally defined as image data indicating multiple densities. FIGS. 12A through 12D show an example showing configuration of a patch image with a black-and-white output engine, wherein the patch image with four density types is defined as primary scanning direction: H pixels, secondary scanning direction: V pixels. Note that with patch images defined with a color output engine, there is a patch image for each component making up the color image. As a representative example, for each of the components of cyan (hereafter, C), magenta (hereafter, M), yellow (hereafter, Y), and black (hereafter K), there are multiple patch images with predetermined densities.
At the time of calibration operation, the patch image is image-formed at a predetermined position on the photosensitive drum. FIG. 13 is an example in the case that a patch image is formed as to the photosensitive drum. Unlike the time of normal printing, the patch image 1302 formed on the photosensitive drum 1301 is not transferred to a transfer sheet, and the density value thereof is read by a sensor 1303 shown in FIG. 13 (step S403). If the density value read by the sensor 1303 is the density value shown by the original patch image, there is no problem in particular, but with an electrophotography method, a density feature at the point-in-time that the printing of the predetermined number of sheets has ended generates density shifting. For example, there are cases wherein the read density value of the patch image with density: 64 greatly overruns or falls below 64. The width of the density shift differs depending on environmental effects such as temperature and humidity inside and outside of the printing apparatus, and how much toner is consumed to print the predetermined number of sheets. That is to say, the density shift width of the patch image differs with the case where image data that is close to a solid is printed on white sheets the predetermined number of sheets, and the case where image data that has a relatively high ratio of white portion is printed on the predetermined number of sheets.
The calibration with the electrophotography method is to correct such density shifting, and is based on creating a correcting table to correct the density shift based on the density values read by the sensor 1303 (step S404).
The concept of creating the correcting table is shown in FIGS. 14A through 14C. FIGS. 14A through 14C take the density level of the patch image as the horizontal axis and the output density level read with the sensor 1303 of the patch image image-formed on the photosensitive drum as the vertical axis. The gridlines of the density level which is the horizontal level shows the density level of the patch image, and with the present description, indicates that there are four patch image densities which are 64, 128, 192, and 255. Also, the four points of Pa, Pb, Pc, and Pd indicate values wherein the reading values of the sensor 1303 in the event of image-forming the patch image on the photosensitive drum as a density reduced value.
In the case of an ideal density feature, the patch image density and the reading density by the sensor 1303 are the same value, and thereby are in a linear density relation as shown by W1. However, in many cases, the density feature after printing a predetermined number of sheets becomes as shown in Pa through Pd in FIG. 14A, and are not plotted on W1, but have output density values with shifts for each of the patch images. With the example in FIG. 14A, each patch image has a higher output density level value than the original density value on the W1. In other words, even with the same image data, the output density after printing the predetermined number of sheets which is the calibration processing timing becomes higher than the image data density at the time of printing starting. Note that the density levels other than the patch image density level, e.g. output density levels corresponding to density levels such as density levels 65 through 127, or 129 through 191, often have performed correcting processing using a measurement output density level near the density level value thereof. That is to say, an output density level equivalent to the density levels of density levels 65 through 127 perform correcting processing using the values of Pa and Pb. A method for correcting processing may be a one-dimensional linear correcting processing, or may be an interpolating processing using a quadratic term. With such interpolating processing, correlation of the density level after the predetermined number of sheets and the output density level come to have the features shown by W2.
Receiving this result, the controller of the printing apparatus creates an inverse converting table W3 such that the output density feature which becomes W2 after printing of the predetermined number of sheets becomes the output density feature of W1. Employing the density feature table of W3 to correct the image data input in the printing apparatus, a linear density image shown in FIG. 14B, i.e. the output image on W1 is obtained.
However, at the point-in-time that the inverse converting table W3 is set, even if the density feature becomes the output feature of the linear density W1, in the case that further printing is performed for a predetermined number of sheets thereafter, density shifting occurs again (FIG. 14C).
FIG. 15 shows the degree of density shifting of the output image data and passage of time in the case that time is shown on the horizontal axis and the output density level is shown on the vertical axis. In FIG. 15, Ps shows the state immediately following calibration, and Pe shows the state wherein printing of a predetermined number of sheets has ended (timing T0). Also, Pr shows the state wherein the calibration operation performed during job execution is completed (timing T1).
As shown in FIG. 15, a shift occurs in the output density between the state Ps immediately following the calibration operation and the state Pe of the timing (T0) wherein printing is finished for the predetermined number of sheets. That is to say, a clear density shift exists in the printed image quality immediately following calibration and the printed image quality immediately preceding the next calibration. The degree (slope) of the density shift thereof depends on the features of the image data printed therebetween. That is to say, the density shift width differs with the case of printing many images with a high image density such as a solid image and the case of printing many images with a low image density having a large portion of white area such as a text image.
Thus, processing to correct such density shifting for each printing of predetermined number of sheets is calibration processing, but because of the processing configuration thereof, the processing must be performed by stopping the job currently having printing executed. Stopping a job during printing executing directly relates to decreased productivity of the printing apparatus. In order to increase productivity, extending the spacing between performing calibration processing, i.e., increasing the predetermined number of printing sheets increases productivity. However, the output image quality immediately following calibration and the output image quality immediately preceding the next calibration has a large image quality difference (density difference).
With a printing apparatus that creates high quality printed material and so forth in the “digital business printing” market, improved productivity and high quality image output are not simultaneously required. Also, in many cases, a printing apparatus for the “digital business printing” market has the functionality to exceed 100 sheets of printed sheets per minute, and a printer engine is employed wherein image quality and high quality image output are enabled.
In order to obtain high quality image output, density shifting should be suppressed as much as possible. That is to say, frequently performing calibration processing is ideal. However, while executing calibration processing, the job during printing output executing must be stopped, whereby the function of a POD high-speed device is not satisfied. Also, even if calibration processing is performed when printing every predetermined number of sheets, in the case of the POD high-speed device, the number of printed sheets per minute is high, so even if the predetermined number of sheets is 2000 sheets, calibration processing is performed once before 20 minutes have passed.
As to decreased productivity due to the calibration processing, Japanese Patent Laid-Open No. 2004-142163 discloses switching the density correcting processing based on the environmental temperature/humidity information inside and outside the apparatus. FIG. 16 shows a diagram of performance timing of the calibration processing with Japanese Patent Laid-Open No. 2004-142163. Selectively performing calibration with patch image output, or performing correction data generating based on simulation at the host side that is connected to the printing apparatus, at the time of environmental temperature/humidity change (Tk), is described. Productivity is increased by decreasing the number of times of calibration processing by patch image output at the printing apparatus side resulting from environmental temperature changes inside and outside the apparatus.
Also, Japanese Patent Laid-Open No. 11-177822 discloses changing timing of calibration processing according to the image output operation mode set by the user who uses the printing apparatus. FIG. 17 shows a diagram of performance timing of the calibration processing with Japanese Patent Laid-Open No. 11-177822. At calibration timing (T0) and thereafter, in the case that an operation mode is selected by the user which requires less frequent calibration processing of the patch image output, the next calibration timing (T1) is shifted later temporally. Thus, the number of times of calibration processing by patch image output at the printing apparatus side resulting from environmental temperature changes inside and outside the apparatus is decreased, thereby increasing productivity.
Japanese Patent Laid-Open No. 11-164148 discloses the correlation between the image output mode set by the user who is using the printing apparatus, the calibration processing, and the confirmation of predicted image quality. Specifically, description is given to show that the calibration processing can confirm the image quality of the job final image before executing the job, upon recognizing that calibration processing will start during job execution. FIG. 18 shows timing to perform calibration and timing to confirm virtual image quality with the Japanese Patent Laid-Open No. 11-164148.
Japanese Patent Laid-Open No. 11-164148 is described with the premise of the case that there are three types of operation modes of the calibration processing of the printing apparatus which are high-speed: every 300 sheets, standard: every 150 sheets, and high precision: every 50 sheets. In the case that the operation mode of the calibration processing is standard mode at the point-in-time of the user using the printing apparatus, and the number of sheets to be input is 200 sheets, calibration processing is executed after 150 sheets are printed (T0). Accordingly, the final image data of the job becomes image data at the point-in-time that 50 further sheets are printed after linear density correcting is performed by the calibration processing (Tjbe). Japanese Patent Laid-Open No. 11-164148 holds a density shift computing coefficient at the point-in-time of one sheet printed, and predicts and displays output image quality by proportion calculations. That is to say, with the previously described example, the coefficient value and the 50 sheets after executing calibration processing are multiplied, and a predicted output image is created. Upon the user confirming the predicted output image quality, selection/switching of the operation mode (high-speed/standard/high precision) of the calibration processing of the printing apparatus is performed.
However, with the current technology disclosed in Japanese Patent Laid-Open No. 2004-142163, reducing the number of times of calibration operation by the patch image output is mentioned, but for correcting the density shift, the focus thereof is only on parameters of the environment temperature/humidity. However, as described previously, important factors for the density shift of the output image after the predetermined number of sheets printed are not only environment temperature/humidity but also elements from the image density. Also, what sort of correcting table will be created to perform correcting processing in accordance with the parameters of the environment temperature/humidity is also not mentioned.
Also, with the current technology disclosed in Japanese Patent Laid-Open No. 11-177822, calibration spacing is variable in accordance with the operation modes of the printing apparatus. By extending calibration spacing, productivity increases, but the spacing to perform density shift correcting is extended, whereby actual output image quality does not move in the direction of achieving high image quality output, but moves in the direction to permit deterioration.
The current technology disclosed in Japanese Patent Laid-Open No. 11-164148 is an assistive function for the user to set the calibration operation mode before job execution, and is not related to correcting processing to improve output image quality. Also, the final output image quality of the job is predicted but parameter used for prediction is a constant, and influences from image data to be printed until the calibration is not considered.
That is to say, none of the current technologies described above offer solutions to the problem needed particularly for a POD high-speed device to achieve both high productivity and improved output image quality.